The present invention relates to color video cameras and more particularly to improvements in the white balance control apparatus for color video cameras.
The white balance adjustment of a color video camera can be performed automatically by sensing changes in the color temperature of an illuminating light by means of sensors of the R, G and B light components which are independent of the area imaging device of the video camera and have substantially the same characteristics as the R (red), G (green) and B (blue) imaging characteristics of the video camera, and by controlling a part or the whole of the gains of the R, G and B color signals in the video camera.
This method of separately providing light sensors for detecting changes in color temperature can automatically perform the white balance adjustment continuously or at any desired time in response to a continuous change in the color temperature of an illuminating light without using any specific white or achromatic object and without interrupting the shooting in progress, and the method has particularly important merits when used with portable video systems.
However, this method has the following disadvantages. In other words, the luminous intensities which can be measured by the light sensors (the photodiodes, silicon cells, etc.) have a limitation which is dependent on the signal-to-noise ratio so that when the absolute intensity of the light illuminating the object decreases, the measured value of the light tends to become extremely unreliable. (For example, a commercially available color temperature meter is provided with a note that its measured value cannot be assured if the luminous intensity is lower than a certain value.) As a result, the above-mentioned method of controlling the gains of the color signals in the video camera by means of the light sensors naturally has a limitation with respect to the luminous intensities and thus the white balance adjustment cannot be effected at illumination levels lower than a certain intensity. Usually, night scenes and candle-lit scenes have luminances which are substantially lower than that which can be measured by the light sensors and therefore the above-mentioned method of separately providing light sensors cannot be used for shooting such scenes.